Userland: Add LibSystem and funnel all syscalls through it

This achieves two things:

- Programs can now intentionally perform arbitrary syscalls by calling
  syscall(). This allows us to work on things like syscall fuzzing.

- It restricts the ability of userspace to make syscalls to a single
  4KB page of code. In order to call the kernel directly, an attacker
  must now locate this page and call through it.

Userland/Libraries/LibC/unistd.cpp

@@ -27,7 +27,6 @@
 #include <AK/ScopedValueRollback.h>
 #include <AK/String.h>
 #include <AK/Vector.h>
-#include <Kernel/API/Syscall.h>
 #include <alloca.h>
 #include <assert.h>
 #include <errno.h>
@@ -42,6 +41,7 @@
 #include <sys/ioctl.h>
 #include <sys/mman.h>
 #include <sys/types.h>
+#include <syscall.h>
 #include <termios.h>
 #include <time.h>
 #include <unistd.h>